A known vehicle door in which a door frame constituting a window frame is fixed to a door body (door panel) provided at a vehicle body, the door body being selectively opened and closed relative to the vehicle body is disclosed in JP2010-120571A (i.e., hereinafter referred to as Patent reference 1), for example. The known door frame includes a pillar portion fixed to an upper end portion of the door body and extending in an upward direction of a vehicle, and a main frame portion connected to an upper end of the pillar portion and extending in a forward or rearward direction of the vehicle to be connected to the upper end portion of the door body.
A vacuum force in an outward direction of a vehicle acts on a vehicle door by a negative pressure generated by an airflow along the vehicle door, particularly, during a high speed running state of the vehicle. Further, because the door frame is fixed to the upper end portion of the door body, the vacuum force is applied to the door frame to bend towards an outward of the vehicle with a bottom end of the door frame as a fixed end. Because the main frame portion connects the door body and the upper end of the pillar portion and has a configuration which inclines in a front-rear direction of the vehicle, the main frame portion is twisted by the vacuum force to be bent towards the outward of the vehicle. Thus, it is desirable that the main frame portion has high rigidity against a torsional force. In many cases, an envelope portion forming a framework of the main frame portion includes a closed cross-section, for example, a cross-section formed by four bars formed in a rectangular shape (e.g., Patent reference 1).
Further enhancement of the rigidity of the door frame for the vehicle has been required. However, according to the construction disclosed in Patent reference 1, because the envelope portion of the main frame portion is formed in the closed cross-section, in order to enhance the rigidity of the main frame portion, a thickness of the main frame portion needs to be increased, which may cause a significant increase in weight of the door frame.
A need thus exists for a door frame for a vehicle which is not susceptible to the drawback mentioned above.